In the same way that different carrier frequencies are used to provide many radio communication channels over the same airways, different wavelengths of light can also carry different channels over the same optical fiber. Unfortunately, in optical systems there is no simple analogous device to the electronic tuned circuit for channel selection. The available optical devices are costly to manufacture, tend to use expensive materials, and frequently lack in performance.
In waveguide structures, frequency selective elements are usually fabricated using some form of integrated diffraction gratings. Such elements are generally quite large and difficult to make. Frequently, gratings are fabricated using electron beam or holographic lithography along the length of a waveguide. A single grating acts as a band-stop filter, reflecting a band of wavelengths that match the grating spacing, and transmitting the rest. (e.g. U.S. Pat. No. 5,416,866 to Sahlen et. al.) A narrow band-pass filter can be constructed using two such filters shifted by an integral number of half-waves. Such a filter rejects a relatively wide band where each of the gratings is reflective, and transmits a narrow band that corresponds to the spacing between the two grating mirrors. Since the grating period must be on the order of the optical wavelength in the material, fabricating such devices requires complex sub-micron lithography.
Planar waveguide structures have also been used for wavelength selection. In these devices the light travels along a two dimensional surface and particular wavelengths are diffracted at different angles. The most common implementations require either a lithographic grating in a Rowland circle configuration "Monolithic InP/InGaAsP/InP grating spectrometer for the 1.48-1.56 mm wavelength range," Applied Physics Letters, vol. 58, p. 1949-51, 1991), or a waveguide phased-array. ("Integrated optics NxN multiplexer on silicon," C. Dragone et al., IEEE Photonics Technology Letters, vol. 3, p. 396-399, 1991.) Compared to single element gratings, these devices can filter a number of wavelengths simultaneously, however, the consume a large rectangular area and also require precision lithography.
A fundamentally different kind of filter relics on wavelength-sensitive coupling between two waveguides. In a directional coupler, two waveguides are fabricated close to each other to allow evanescent field coupling between the two guides. If the waveguides are similar then each wavelength of light will have the same propagation constant in both waveguides, and light will completely couple from one waveguide to the next. By breaking the symmetry in such a directional coupler, wavelength selectivity can be obtained. With dissimilar waveguides, the propagation constants are different in the two waveguides at all wavelengths except for one, and only that wavelength will couple between the guides. Wavelength sensitive behavior in such asymmetric waveguide couplers is generally well known (U.S. Pat. No. 3,957,341), but their use has been limited since the devices are long and often do not have the required wavelength resolution. If device size can be reduced from many millimeters to hundreds of microns, it becomes possible to cascade devices in series for multiple wavelength operation. As one might expect, the more dissimilar the two materials in the waveguides, the higher the resolution and the shorter the length, but in most material systems one cannot vary the refractive index of the materials for the waveguides by large amounts. For example, in the lithium niobate system, waveguides are formed by diffusing titanium to form high index regions and such a coupler has been demonstrated (U.S. Pat. No. 4,146,297) with a relative wavelength resolution (.delta..lambda./.lambda.) of 1/30 at a device length of 1.5 cm. ("Tunable optical waveguide directional coupler filter," R. C. Alferness et al., Applied Physics Letters, vol. 33, p. 161-163, 1978). In semiconductors, a higher refractive index variation is obtainable and a relative wavelength resolution of 1/850 can obtained with a length of 5 mm "InGaAsP/InP vertical directional coupler filter with optimally designed wavelength tunability", C. Wu et al., IEEE Photonics Technology Letters, vol. 4, p. 457-460, 1993). The wavelength selectivity and device length trade-off can be expressed as: ##EQU1## where L is the device size, .delta. is the wavelength, .delta..lambda. is the wavelength resolution, and n.sub.1 and n.sub.2 are the refractive indices of the low index and high index waveguides respectively.
In addition to size, many optical filters are costly since they require rare and expensive compound semiconductor materials. These compound semiconductors are usually lattice matched alloys, typically indium phosphide or gallium arsenide based, where the composition of the material can be varied during crystal growth to make a variety of waveguiding structures. In contrast to the compound semiconductors, silicon is much lower cost and can be obtained in large standard wafers, but waveguides made on silicon using various glasses do not have the required range of refractive indices for asymmetric waveguide filters.
U.S. Pat. No. 3,957,341 describes the basic operating characteristics of an asymmetric directional coupler, and how the device can be used as an optical filter.
U.S. Pat. No. 4,146,297 explains how the asymmetric directional coupler can be implemented in the lithium niobate material system, and how the addition of electrodes can provide tuning.
U.S. Pat. No. 5,234,535 describes a process where silicon-on-insulator substrates can be prepared.
U.S. Pat. No. 5,343,542 by Kash describes a device wherein different wavelengths of light are filtered from a waveguide sequentially.
U.S. Pat. No. 5,416,866 teaches how diffraction gratings can be fabricated in waveguides and specifically shows how two gratings used in sequence can the increase tuning range of a filter.